fix: run manifest containers as root and drop privileges in entrypoint

The compose file was forcing the container to run as uid 1001, so the
entrypoint could not create /workspace even with sudo configured.

- Remove Dockerfile USER directive so containers start as root
- Make compile_compose use user: 0:0 when the manifest declares a user
- Make the entrypoint drop to the container user via  after setup,
  preserving environment variables and command arguments
- Update unit tests to assert root startup and privilege drop

Quality gates:
- pytest tests/unit: 210 passed
- ruff: clean on changed files
- mypy: clean on changed files

apps/api/src/services/build/manifest_compiler.py

@@ -252,9 +252,10 @@ def compile_dockerfile(manifest: dict) -> str:
         lines.append("RUN chmod +x /usr/local/bin/headquarter-entrypoint")
         lines.append("")
 
-    # Switch to runtime user
-    if user:
-        lines.append(f"USER {user['name']}")
+    # Do not switch to the runtime user in the Dockerfile. The entrypoint
+    # starts as root so it can create the /workspace compatibility symlink
+    # (which lives under /) and fix mount ownership, then it drops privileges
+    # to the container user before exec-ing the real command.
 
     # Set WORKDIR to the configured home directory unless runtime.working_dir
     # explicitly overrides it.
@@ -373,7 +374,16 @@ def compile_entrypoint(manifest: dict) -> str:
         lines.append(script)
         lines.append("")
 
-    lines.append('exec "$@"')
+    # Drop from root to the container user before running the real command.
+    # The Dockerfile no longer sets USER, so the entrypoint has root for the
+    # setup above.  Use `su -c` to preserve environment variables (HOME, PATH,
+    # WORKSPACE_NAME, etc.) and keep the container user as the running user.
+    if user:
+        name = user["name"]
+        lines.append("# Drop privileges to the container user")
+        lines.append(f'exec su -s /bin/bash -c "exec \\"$@\\"" {name} -- "$@"')
+    else:
+        lines.append('exec "$@"')
     return "\n".join(lines)
 
 
@@ -414,9 +424,12 @@ def compile_compose(manifest: dict, variables: dict[str, Any]) -> str:
             runtime["working_dir"], home_dir
         )
 
-    # User override
+    # The entrypoint starts as root (Dockerfile does not set USER) so it can
+    # create the /workspace compatibility symlink and fix mount ownership. It
+    # drops privileges to the container user internally before exec-ing the
+    # real command, so do not set compose-level user override here.
     if user:
-        service["user"] = f"{user['uid']}:{user['gid']}"
+        service["user"] = "0:0"
 
     # Ports for web tools
     default_port = manifest.get("default_port")